Several up-stream processes are required before a complex biological fluid can be analyzed for analytes. For example, to perform HIV viral load detection, the separation of plasma from blood is the first up-stream step since hemoglobin and blood cells interfere with the subsequent amplification and detection of viral RNA. The separation of plasma is also a critical upstream process for the detection and diagnosis of infectious diseases. For example, the detection of HIV in adults using HIV-specific antibodies or the detection of HIV in infants by using an HIV core p24 protein require the separation of plasma from whole blood.
In a laboratory setting, the separation of plasma from whole blood is carried out by centrifugation of blood for 20 minutes at 3000 g. In doing so, the solid components of blood settle down in the sediment and the supernatant liquid consists of plasma. This protocol usually requires a trained technician to manually pipette out the supernatant for further analysis. While large scale automated sample preparation systems can eliminate the manual step, these instruments are expensive instrumentation, making them unsuitable for resource limited or point-of-care testing.
Methods have been designed to integrate the centrifugal blood separation with further downstream steps through a micro-fluidic platform (Brenner et al. Special Publication-Royal Society of Chemistry 2004, 296:566-568, Haeberle et al. Lab on a Chip 2006, 6:776-781, Kang et al. Special Publication-Royal Society of Chemistry 2004, 296:614-616, Madou et al. Biomedical Microdevices 2001, 3:245-254, Toner & Irimia. Annual Review of Biomedical Engineering 2005, 7:77-103, Luo et al. J Clin Microbio/2005, 43:1851-1857, herein incorporated by reference in its entirety). However, these methods work with an extremely limited volume of whole blood, require the use of an instrument to create the centrifugal force, are prone to clogging, and/or achieve only limited purity. The use of synthetic membranes to separate blood from plasma avoids some of the problems presented by centrifugation and microfluidics systems; however, devices are complex due to the need for multiple filter layers (Vogel et al. Boehringer Mannheim GmbH; 1984, Vogel et al. Boehringer Mannheim GmbH; 1989, herein incorporated by reference in their entireties), are inherently fragile and difficult to use do to the membrane materials used (e.g., glass fiber), comprise materials which interfere with assays (Baumgardner & Loewen. Ahlstrom Filtration, Inc.; 1993, Loewen & Baumgardner. Tappi Journal 1996, 79:183-184, herein incorporated by reference in their entireties), and contain materials which retard the flow of blood into the filters (Suzuki & Ho: A magnetic force driven chaotic micro-mixer. pp. 40-43; 2002: 40-43, herein incorporated by reference in its entirety).